Absorption refrigeration system

ABSTRACT

An absorption refrigeration system employing a generator having a heat pipe associated therewith to transfer heat from a suitable burner to the interior of the generator to increase the capacity of the generator without a proportionate increase in the size thereof and to minimize the inside surface temperature of the generator wall.

United States Patent Patel Sept. 12, 1972 54] ABSORPTION REFRIGERATION3,254,507 6/1966 Whitlow ..62/497 X SYSTEM 3,520,282 7/1970 Fisher..62/497 x Inventor: Jashwant D. Patel, 1025 James St.

Apt. 32, Syracuse, NY. 13203 Filed: Feb. 22, 1971 Appl. No.: 117,577

US. Cl. ..62/476, 62/497, 122/33 Int. Cl. ..F22b 1/02, F25b 15/04 Fieldof Search ..62/148, 476, 497; 122/33 References Cited UNITED STATESPATENTS 9/1864 Bayley ..122/33 12/1891 Hill ..122/33 X PrimaryExaminer-William F. ODea Assistant Examiner-4. D. FergusonAttorney-Harry G. Martin, Jr. and J. Raymond Curtin [5 7] ABSTRACT Anabsorption refrigeration system employing a generator having a heat pipeassociated therewith to transfer heat from a suitable burner to theinterior of the generator to increase the capacity of the generatorwithout a proportionate increase in the size thereof and to minimize theinside surface temperature of the generator wall.

4 Claims, 2 Drawing Figures PATENTEDSEP 12 I972 w'uuuu INVENTOR.JASHWANT D. PATEL BY W ATTORNEY ABSORPTION REFRIGERATION SYSTEMBACKGROUND OF THE INVENTION In direct fired absorption refrigerationmachines, it is difficult to transfer heat from the flue gas to thesolution within the generator without heating the generator wall to atemperature which permits corrosion of the generator and deteriorationof the solution therein. For optimum machine efficiency the greatestquantity of heat possible should be transferred from the flue gas to thesolution.

One method for accomplishing this purpose is to provide fins on thegenerator to transfer the heat to the generator wall and subsequently tothe solution therein. However, the combustion temperature adjacent theburner is often high enough to oxidize the fins or to produce anexcessive generator wall temperature which could accelerate corrosion ofthe generator and cause premature failure thereof.

Another method for transferring a large quantity of heat to the solutionwithin the generator is to employ a heat pipe, a portion of which isexposed to the burner and a portion of which is exposed to the solutionwithin the generator. The heat pipe is charged with a suitable heatexchange fluid such as water. The heat transfer fluid within the heatpipe is vaporized by the heat from the burner. The heat of vaporizationis transferred to the solution within the generator, thereby condensingthe heat transfer fluid. The condensed heat transfer fluid passes intothe portion of the heat pipe adjacent the burner where it is againvaporized for passage into the interior of the generator.

SUMMARY OF THE INVENTION This invention relates to an aqua-ammoniaabsorption refrigeration system having a generator, an absorber, acondenser and an evaporator connected to provide refrigeration. Burnermeans are provided adjacent the generator for heating the solutiontherein. A heat pipe having heat exchange medium therein is associatedwith the generator, the heat pipe comprising an interior coil within thegenerator in contact with the solution therein and an exterior coilencircling the generator in the discharge path of the flue gas from theburner, the lowest turn of the coil within the generator being offset inan upward direction from the lowest turn of the coil exterior of thegenerator for increased circulation within the heat pipe, the interiorand exterior coils extending helically in the same circular direction toprovide a smooth transition at the top of the coils for the flow of heatexchange medium from the exterior coil to the interior coil.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of anabsorption refrigeration system employing a generator having a heat pipeassociated therewith.

FIG. 2 is a sectional view of the generator illustrated in FIG. 1 takenalong line II-II of FIG. 1.

i DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawingsthere is illustrated an absorption refrigeration system comprising anabsorber 10, a condenser 12, an evaporator or chiller 14, a

generator 16, a liquid-suction heat exchanger 18, and a vapordistributor 20 connected to provide refrigeration. A pump 22 is employedto circulate weak absorbent solution from absorber 10 to generator 16.

As used herein, the term weak absorbent solution refers to solutionwhich is weak in absorbent power, and the term strong absorbent solutionrefers to a solution which is strong in absorbent power. A suitableabsorbent for use in the system described is water; a suitablerefrigerant is ammonia.

Liquid refrigerant condensed in condenser 12 passes through refrigerantliquid passage 24 to the liquid-suction heat exchanger. Theliquid-suction heat exchanger 18 includes a housing 26 having arefrigerant restrictor 28 at the upstream end and a refrigerantrestrictor 30 at the downstream end thereof. A portion of the liquidrefrigerant supplied to the liquid-suction heat exchanger 18 flashesupon passing through restrictor 28 due to the low pressure existingdownstream of the restrictor, thereby cooling the remainder of therefrigerant in the housing 26. The cooled refrigerant liquid and flashedrefrigerant vapor then pass through restrictor 30 into heat exchanger 32of chiller 14.

A heat exchange medium such as water is passed over the exterior of heatexchanger 32 where it is chilled by giving up heat to evaporate therefrigerant within the heat exchanger. The chilled heat exchange mediumpasses out of chiller 14 through line 34 to suitable remote heatexchangers (not shown) after which it is returned to the chiller throughinlet 36 for rechilling.

The cold refrigerant evaporated in heat exchanger 32, along with a smallquantity of absorbent which is carried over to the chiller with therefrigerant from the generator, passes into refrigerant vapor passage 38of liquid-suction heat exchanger 18. The refrigerant vapor and absorbentliquid, which has a large quantity of refrigerant absorbed therein,passes through refrigerant vapor passage 38 in heat exchange relationwith the refrigerant passing through housing 26. Refrigerant vapor andabsorbent solution from passage 38 passes to refrigerant distributor 20through line 42. The refrigerant vapor and absorbent solution from line42 are mixed in the distributor 20 with solution from the generator 16which is supplied to the distributor through line 44.

The absorbent solution-refrigerant vapor mixture from distributor 20 issupplied to absorber 10 where a cooling medium, preferably ambient air,is passed over the surface of the absorber in heat exchange relationwith the solution therein for cooling the absorbent solution to promotethe absorption of the refrigerant vapor by the solution. The samecooling medium may be supplied to condenser 12 in heat exchange relationwith refrigerant vapor therein to condense the refrigerant.

Cold weak absorbent solution passes from absorber 10 through line 46into the pump 22. Liquid from pump 22 is passed through line 48 torectifier heat exchange coil 50. The weak solution passes through coil50 in heat exchange relation with hot strong solution passing throughheat exchange coil 52 disposed within coil 50 and with the hotrefrigerant vapor flowing through rectifier shell 54 in contact with theouter surface of coil 50. The weak solution from coil 50 is dischargedinto the upper portion of generator 16 along with any vapor which isformed in coil 50 due to heat exchange with the hot vapor passingthereover and the hot solution flowing therethrough.

Generator 16 comprises a shell 56 having fins 58 suitably affixedthereto as by welding. The generator is heated by a gas burner 60 orother suitable heating means. A heat exchanger or heat pipe 62comprising a coil 64 external of the generator and a coil 66 on theinterior of the generator is provided to transfer heat from the burner60 to the solution within generator 16. The heat pipe 62 is formed of asuitable material such as stainless steel which is resistant tocorrosion at the high temperature produced by the burner 60. The heatpipe is partially filled with a suitable heat exchange medium such asdistilled water. In areas where the machine might be exposed to freezingtemperatures, a suitable anti-freeze solution such as a 12 percentaqueous lithium bromide solution could be used in place of distilledwater. The heat from burner 60 vaporizes the heat exchange medium incoil 64. The vapor from coil 64 passes through fitting 70 into coil 66within the genera tor where it is condensed by giving up heat to thesolution in the generator. The condensed heat exchange medium from coil66 passes through drain line 68 to coil 64 where it is again vaporizedfor subsequent passage through fitting 70 into coil 66.

The heat pipe configuration and the quantity of liquid heat exchangemedium therein is critical to the proper operation of the heat pipe. Forsafe operation, the pressure developed within the sealed heat pipe mustbe carefully controlled. An insufiicient quantity of heat exchangemedium will allow the vapor generated therein to be superheated, therebycausing a rapid rise in pressure beyond safe operating limits. Anexcessive charge of heat exchange medium will also result in internalpressures beyond the desired maximum pressure. For optimum performanceof the heat pipe, the vapor fraction of the heat exchange medium atfitting 70 should be 100 percent. For a given heat pipe tube length, therate of heat exchange medium circulation within the pipe is dependentupon tube diameter, the degree of upward displacement of the interiorcoil in relation to the exterior coil of the heat pipe and therestriction to flow caused by the innerconnection between the exteriorand interior coils at the upper portion thereof. To reduce the flowrestriction at the top portion of the heat pipe, the interior andexterior coils extend helically in the same circular direction, therebyproviding a smooth transition from the exterior coil to the interiorcoil at the top of the heat pipe. The degree of offset between theinterior and exterior coils is limited by the internal construction ofthe generator and the maximum desired installed height of the generatorwithin the machine housing. By arranging the lowest turn of the interiorcoil approximately one inch above the lowest turn of the exterior coil,the desired circulation rate of heat exchange medium from the interiorcoil to the exterior coil is obtained.

The diameter of the heat pipe tubing also effects circulation of heatexchange medium within the coil. By constructing the heat pipe of%-inch-diameter tubing, the desired circulation of heat exchange mediummay be obtained. While it is possible to construct the heat pipe out ofsmaller or larger tubing, smaller tubing may restrict the circulation ofheat exchange medium within the heat pipe, creating the possibility ofexcessive pressures being generated therein. Tubing larger thanfiveeighth inch, while it does not restrict the flow of heat exchangemedium, is undesirable since the sharp bends necessary in the restrictedspace above the burner and within the generator are difficult anduneconomical to manufacture.

As stated heretofore, the charge of heat exchange medium within the heatpipe is critical. In a heat pipe constructed in accordance with theforegoing parameters, a minimum charge of approximately 35 percentshould be utilized to obviate the possibility of vapor superheat and theexcessive pressures caused thereby. A volume of liquid heat exchangemedium exceeding 65 percent of the total interior volume of the heatpipe may also cause excessive pressures to be generated within the heatpipe. The volume of liquid heat exchange medium charged into the heatpipe should therefore be within the range of 35 percent to 65 percent ofthe interior volume of the heat pipe.

Since the heat pipe is utilized to increase the amount of heat which maybe transferred from a burner to the solution within an absorptionrefrigeration system generator while maintaining the temperature of thegenerator walls below the temperature level at which acceleratedcorrosion occurs, it is also necessary to maintain the exterior surfaceof the interior coil of the heat pipe below the temperature level atwhich accelerated corrosion results. To obtain this wall temperature,the heat transfer surface of the heat pipe in contact with the solutionshould be approximately onehalf the surface area of the heat pipe incontact with the flue gases from the burner.

Thus by providing a heat pipe having a %-inch-tube diameter, a heattransfer surface within the generator approximately one-half theexterior heat transfer surface, an offset of approximately one inchbetween the bottom of the interior coil and the bottom of the exteri-'or coil, an interior coil wound in the same direction as the exteriorcoil and a charge of heat transfer medium within the range of 35 percentto 65 percent of the total volume of the interior of the heat pipe,maximum transmission of heat from the burner to the solution may beobtained while maintaining internal pressures within the heat pipewithin a safe range and maintaining the exterior wall temperature of theinterior coil of the heat pipe below the temperature at whichaccelerated corrosion by the absorbent solution is encountered.

By locating the outer coil of the heat pipe in close proximity to thelower surface of the generator vessel, the heat from the burner istransferred both to the generator wall and to the solution within theheat pipe. Thus, the flue gas from the burner contacts a large heatexchange surface which in effect lowers the temperature of the heatexchange surface while transferring more heat thereto. The sameconditions prevail on the interior of the generator. Since heat istransferred to the solution through the generator wall and through thewalls of the heat pipe coil, a greater quantity of liquid is exposed tothe increased heat transfer surface, allowing a greater quantity of heatto be transferred to the solution.

The temperature of the solution within the generator is ordinarily afunction of the boiling point of the solution at the pressures existingwithin the machine. However, since a large quantity of heat must betransferred to the solution, the solution nearest the heat transfersurface may be heated to a temperature substantially above the boilingpoint. By increasing the heat exchange surface area within thegenerator, a greater quantity of heat may be transferred to the solutionwhile maintaining the solution adjacent the heat exchange surface at atemperature nearer the boiling point of the solution than has heretoforebeen possible. Therefore, by utilizing the heat pipe, the generator walltemperature and the temperature of the solution within the generator isreduced, while at the same time the flue gas temperature discharged fromthe generator is also reduced. Thus, a greater quantity of heat issupplied to the solution than has been possible heretofore even thoughthe maximum temperature of the solution and the generator wall ismaterially reduced.

The weak solution which is boiled in generator 16 concentrates thesolution, thereby forming a strong solution and refrigerant vapor.

The hot strong absorbent solution passes upwardly through the analyzersection of generator 16, through analyzer coil 76 in heat exchange withthe weak solution passing downwardly over the coil. The warm strongsolution then passes through heat exchange coil 52 within coil 50 andline 44 into the distributor 20. A restrictor 78 is provided in line 44so that the solution supplied to the vapor distributor 20 is at the samepressure as the vapor in line 42.

Refrigerant vapor formed in generator 16 passes upwardly through theanalyzer section thereof where it is concentrated by mass heat transferwith weak solution passing downwardly over analyzer coil 76. Analyzerplates 80 in generator 16 provide a tortuous path for flow of solutionand vapor to assure intimate contact therebetween to improve the massheat transfer. The vapor then passes through rectifier 54in heatexchange relation with the weak solution passing through coil 50.Absorbent condensed in rectifier 54 flows downwardly into the generatoralong with the weak solution discharged from coil 50. Refrigerant vaporpasses from rectifier 54 through line 82 to condenser 12 to complete therefrigeration cycle.

While I have described a preferred embodiment of my invention, it is tobe understood that the invention is not limited thereto since it may beotherwise embodied within the scope of the following claims.

I claim:

1. An aqua-ammonia absorption refrigeration system comprising agenerator, an absorber, a condenser and an evaporator connected toprovide refrigeration;

burner means disposed adjacent said generator for heating the solutiontherein;

a heat pipe associated with said generator having a heat exchange mediumtherein, said heat pipe having a heat exchange coil interior of thegenerator in contact with the solution therein and a heat exchange coilexterior of and encircling the generator in the discharge path of theflue gas from said burner means, the lowest turn of said coil withinsaid generator being offset in an upward direction from the lowest turnof said exterior coil for increased circulation within said heat pipe,said interior and exterior coils extending helically in the sa ecircular dire tion to rovide a smoot transiti n at the top of said cor sfor the flow o heat exchange medium from the exterior coil to theinterior coil, the offset between the coil within said generator and thecoil exterior of said generator and the smooth transition between theexterior and interior coil providing a minimal restriction to the flowof heat exchange medium to increase heat transfer efficiency and toreduce pressure within the coil.

2. An absorption refrigeration system according to claim 1 wherein theheat exchange surface area of said exterior coil is substantially doublethe surface area of said interior coil.

3. An absorption refrigeration system according to claim 2 wherein saidheat exchange pipe coils are formed of approximately five-eighth inchoutside diameter steel tubing, the lowest turn of said interior coilbeing disposed approximately one inch above the lowest turn of saidexterior coil.

4. An absorption refrigeration system according to claim 3 wherein saidheat pipe is charged with a volume of liquid heat exchange medium equalto at least 35 percent and not exceeding 65 percent of the interiorvolume of said heat pipe.

1. An aqua-ammonia absorption refrigeration system comprising agenerator, an absorber, a condenser and an evaporator connected toprovide refrigeration; burner means disposed adjacent said generator forheating the solution therein; a heat pipe associated with said generatorhaving a heat exchange medium therein, said heat pipe having a heatexchange coil interior of the generator in contact with the solutiontherein and a heat exchange coil exterior of and encircling thegenerator in the discharge path of the flue gas from said burner means,the lowest turn of said coil within said generator being offset in anupward direction from the lowest turn of said exterior coil forincreased circulation within said heat pipe, said interior and exteriorcoils extending helically in the same circular direction to provide asmooth transition at the top of said coils for the flow of heat exchangemedium from the exterior coil to the interior coil, the offset betweenthe coil within said generator and the coil exterior of said generatorand the smooth transition between the exterior and interior coilproviding a minimal restriction to the flow of heat exchange medium toincrease heat transfer efficiency and to reduce pressure within thecoil.
 2. An absorption refrigeration system according to claim 1 whereinthe heat exchange surface area of said exterior coil is substantiallydouble the surface area of said interior coil.
 3. An absorptionrefrigeration system according to claim 2 wherein said heat exchangepipe coils are formed of approximately five-eighth inch outside diametersteel tubing, the lowest turn of said interior coil being disposedapproximately one inch above the lowest turn of said exterior coil. 4.An absorption refrigeration system according to claim 3 wherein saidheat pipe is charged with a volume of liquid heat exchange medium equalto at least 35 percent and not exceeding 65 percent of the interiorvolume of said heat pipe.